1. Field of the Invention
The present invention relates to a disk discrimination method, and more specifically, to a stable disk discrimination method.
2. Description of the Prior Art
There are many kinds of optical disks, such as CD-ROM, CD-R, CD-RW, Hybrid CD, single-layer DVD, double-layer DVD, DVD-R, DVD+R, DVD-RW, DVD+RW, DVD-RAM, and so on. Because different kinds of optical disks have different data formats and different physical characters, an optical disk drive includes several data-reading methods, like exchanging wavelengths of the laser beams emitted from a pick-up head (CD: 780 nm, DVD: 650 nm) or amplifying some predetermined signals. Therefore, when the optical disk is loaded into the optical disk drive capable of reading various kinds of optical disks, the optical disk needs to be discriminated for determining the next data-reading step corresponding to the optical disk.
Please refer to FIG. 1. FIG. 1 is a diagram of a focus mechanism with a pick-up head of an optical disk drive. A laser diode 1 is used for generating a laser beam, and the laser beam generated by the laser diode 1 is passed through a beam splitter 3 and an object lens 5 to generate a focus point 9. A focus actuator 11 can actuate the object lens 5 to move upward or downward so as to change the location of the focus point 9. The light reflected from the focus point 9 on an optical disk 7 is emitted to a photo detector 13 via the beam splitter 3. The photo detector 13 can generate optical signals, such as focus error signal, RF signal, RF level signal, sub-beam add signal, and so on, for controlling focus of the laser beam.
There are two control methods for controlling the focus actuator 11. One is an open-loop control method, and the other is a close-loop control method. The disk discrimination is relative to the open-loop control method, and the detailed introduction is described as follows.
Please refer to FIG. 2. FIG. 2 is a control functional block diagram of the focus open-loop control method. A host sends a control command to a digital/analog IC 20, and then the digital/analog IC 20 generates a control signal. The control signal can be amplified by a driving IC 22 into a focus-driving signal to drive a focus actuator 24 so that the focus point can move upward (as a direction 19 shown in FIG. 1). At the same time, a photo detector 26 can generate a signal and transmit the signal to an RF amplifier 28 for generating a focus error signal. The distances between each material layers are determined according to the focus error signal.
An optical disk with a single recording layer structure is shown as the optical disk 7 in the FIG. 1. When the pick-up head reads/writes data from the optical disk, the laser beam will reach a reflective layer 15, the material layer for recording data, via a plastic layer 17. The thickness of the plastic layer 17 of a DVD-format optical disk is 0.6 mm while CD-format optical disk is 1.2 mm. Therefore the optical disk can be discriminated whether it is DVD format or CD format according to the distance from the surface of the plastic layer 17 to the reflective layer 15 (i.e. the thickness of the plastic layer 17).
The conventional method for discriminating optical disks is by means of the focus error signal. Please refer to FIG. 3. FIG. 3 is a diagram illustrating a S-curve of the focus error signal and the RF level signal in the prior art. The focus point 9 is approaching a Hybrid CD 38 in a direction 56. The S-curve can be plotted by tracking the focus error signal. When the distance between the focus point 9 and the Hybrid CD 38 approaches infinity, the reflective light is slight so that the focus error signal is also slight. When the focus point 9 is approaching a first reflective layer 42 of the Hybrid CD 38, the focus error signal increases to a positive peak value FE_PK1. And when the focus point 9 reaches the first reflective layer 42, the focus error signal decreases to a zero intersection 52. Afterward the focus point 9 is away from the first reflective layer 42, the focus error signal decreases to a negative trough value FE_BT1 and then increases to the zero to form a S-curve.
As shown in FIG. 3, when the focus point 9 passes through three material layers, a plastic layer 40, the first reflective layer 42, and a second reflective layer 44, the focus error signal includes the S-curves S0, S1, and S2. The distances between the zero intersections 50, 52, 54 of the S-curves S0, S1, S2 are the distances between the material layers.
The optical disks are discriminated according to the distances between the zero intersections of the S-curves in the prior art. However the amplitude of the S-curve of the plastic layer is not so obvious as the ones of the first reflective layer 42 and the second reflective layer 44. Sometimes it is difficult to detect the distance between the plastic layer 40 and the first reflective layer 42, so it is also difficult to discriminate the format of the optical disk. Therefore there is a need to discriminate the format of the optical disk more efficiently.